This month, we've partnered with Bespoke Post to bring you July's Box of Awesome, which will feature an aged cocktail kit. Wondering what an aged cocktail is? Here's the best explanation you're likely to find anywhere.

Walk into a hipster cocktail den, artisanal speakeasy or posh hotel bar, and you're likely to find a special "barrel-aged cocktail" on the menu. Bars across North America have hopped on to this cocktail geek trend, with New York, Portland and San Francisco leading the crowd. The phenomenon is still building up steam.

Very simply put, barrel aging is exactly what it sounds like. A bartender mixes up a several-gallon serving of a cocktail (called "batching") and funnels it into an oak barrel, the same kind used to age wine or bourbon. In fact, bars often score the barrel from wineries or distilleries, though they can also be purchased brand-spanking new online. This cocktail-in-a-barrel is set aside in a dark corner of the basement for anywhere from a couple of months to as long as the bartender damn well pleases — that is, until he or she decides it's "ready."

As with wine or bourbon (or rum or aged tequila or any other aged spirit), what's happening inside involves a complex relationship between barrel and alcohol. As the barrel "breathes" (with changes in temperature and humidity), the alcohol is absorbed and expelled from the wood, extracting tannins, color and flavors. The alcohols soften and gain complexity as they age.

"When it works, everything gets richer, fatter and loses that sharp edge," says Dan Greenbaum, bar manager of The Beagle in Manhattan's East Village. What you end up with, hopefully, is a beverage that tastes very different from when it originally went into the barrel.

Barrel-Aging History 101

When Romans and Greeks started buying wine from the Gauls around 300 BC, they learned the juice stored in those French barrels was much tastier than the vinegar-swill they'd been keeping in clay pots and animal skins up till then. Burgundy's wine was not only more delicious, but easier to store and transport, with far less breakage. By the time Europeans set sail to conquer and colonize Africa and the Americas in the 1600s, barrels were de rigueur for storing wine, rum and much more. The concept of aging spirits on purpose was still 100 or more years away, but the elements were in place. Storing booze in a barrel (preferably one that hadn't first been used to hold salted fish or nails) made it last longer and taste richer, fuller and smoother (or at least not as throat-scrapingly harsh).

"Barrel-aging cocktails is actually new again," Greenbaum explained recently at a seminar (co-sponsored by AskMen) during the annual Manhattan Cocktail Classic. "Before Prohibition, cocktails were often pre-batched and stored in barrels until they were bottled. Bottled cocktails were very popular in the late 1800s and early 1900s (and are again), and bars would advertise their drinks as 'barrel aged' or 'bottled.'"

Flash forward to 2010, and Oregon bartender Jeffrey Morgenthaler (who is most often credited with reviving the trend at the Portland bar Clyde Common) found himself visiting London bar owner and cocktail innovator Tony Conigliaro. "Tony C. was experimenting with aging cocktails in glass jars, and I just thought, what would happen if I put them in barrels?" Like many beautiful accidents, trial and error (and a lot of spilled gin) led to the modern thirst for barrel-aged mixed drinks.

"The sharp edges of a drink are softened, but in a way that doesn't make the drink seem flabby or one-note," says Morgenthaler. "Depending on what the barrel held before you aged a cocktail in it, like delicious whisky, those flavors will also work their way into the drink. Fortified wine, like vermouth, is going to be lightly oxidized, so you get earthy, mushroom-y notes that lend a lot of depth."

The humble oak barrel, credited by many distillers and bartenders for as much as 85% of the flavor found in aged single-malt Scotch whiskies, is critical to the process. Cocktails soften and integrate aging in glass jars, but they gain no flavor complexity. American and French oak are the two most popular woods for barrels, and impart different flavor profiles, with American trending towards coconut, vanillin and herbal notes. "Char" or burn the interior, as is done for bourbon, and you get notes of leather, smoke (duh) and chocolate. Aging your drink inside a used sherry barrel adds sweetness, while used bourbon barrels add spice.

Today, bars around the country are riffing on spirit-forward classic cocktails, and experimenting. The team at New York's Orient Express has added coffee and "other secret ingredients" to a Calvados brandy-based cocktail dubbed the Number 44. At the Beagle, Greenbaum and the rest of the team are experimenting with soaking charred oak barrels in sherry, tonic syrup, Fernet Branca and full-bodied liquids. "Those flavors go into the finished cocktail and work like another ingredient in the drink," says Greenbaum. "We're also doing some Solera-style aging on a White Manhattan."

As with Port and Sherry, Solera aging involves fractional blending of aged and un-aged product across several barrels over time. The result is intended to be rich and deep, but with plenty of unexpected brightness as well.